Recently, according to a tendency toward a high density and a high S/N ratio of magnetic materials, particularly of a recording medium for a video deck and a computer, the size of a particle of magnetic powder has become smaller. In terms of a surface area per unit weight, for example, a magnetic tape in a standard grade for a VHS mode or a .beta. mode employs magnetic powder having BET surface area of 19.about.23 m.sup.2 /g class, a magnetic tape in a high grade class employs magnetic powder having BET surface area of 23.about.28 m.sup.2 /g and a magnetic tape in a super high grade class employs magnetic powder having BET surface area of 29.about.33 m.sup.2 /g class. Further, in the near future, magnetic powder in 35.about.70 m.sup.2 /g class is to be put on the market.
Generally, an S/N ratio of a magnetic recording medium is said to be proportional to a square root of the number of particles of magnetic substance in the recording material related to recording and playing back. Therefore, when magnetic powder in the same weight is coated, the magnetic substance having smaller particle size is more advantageous for the improvement of S/N ratio. However, the surface area of a particle becomes larger in inverse proportion to the square of a particle diameter, therefore, the dispersion of particles becomes difficult sharply as the particle diameter becomes smaller and the stability of dispersion deteriorates.
For the dispersion of magnetic powder, an amount of dispersing agent that is just enough to cover the surfaces of magnetic powder particles should normally be enough but actually, the amount is not enough for the satisfactory dispersion and stability. For this reason, the dispersing agent is considerably excessive amount is added. Dispersing agent not adsorbed to the magnetic metal powder is mixed with a binder in the coated film and then plasticizes the magnetic layer and disturbs the hardening of the binder, thus lowers sharply the mechanical strength, especially the Young's modulus of the magnetic layer. Recently, according to the recent tendency for the long-time-recording on a tape, there is a trend for thinning the total thickness of the tape by using a thin base film. However, since the stiffness of the tape is proportional to the cube of the tape thickness, the tendency for thinning the tape reduces sharply the stiffness of the tape, which deteriorates the running efficiency and the head-touch of a thin tape and finally leads to the deterioration of an S/N ratio. In order to keep the mechanical and physical properties of the thinned tape, especially to keep the stiffness of the thinned tape, there have been an adoption of ultra tensilized base film and a movement toward a high Young's modulus of a magnetic layer. Therefore, the drop in a Young's modulus of magnetic layer caused by excessive dispersing agent and other low molecular weight additives deteriorates sharply the mechnical and physical properties of a thin tape.
There have been disclosed various technologies which may disperse effectively and stably the magnetic powder without deteriorating the mechanical and physical properties of the magnetic layer. For example, Japanese Patent Publication Open to Public Inspection Nos. 94308/1979, 143894/1979 and 92103/1975 (hereinafter referred to as Japanese Patent O.P.I. Publication) disclose the technologies wherein the magnetic metal powder is pre-treated with phosphoric ester derivatives.
Further, Japanese Patent O.P.I Publication Nos. 134899/1976, 51703/1978, 7898/1978 and 46509/1979 disclose the technologies wherein the surface of the magnetic layer is covered by silicone oil.
Further, Japanese Patent O.P.I. Publication Nos. 108902/1975, 97738/1974, 33753/1976, 116114/1978 and 24000/1979 disclose the technologies wherein the surfaces of magnetic powder are treated with anionic surfactant. However, the technologies mentioned above are not considered to be effective for fine-particle magnetic powder, especially for magnetic powder with BET 35.about.40 m.sup.2 /g or more.
Further, Japanese Patent O.P.I. Publication Nos. 103403/1976, 33602/1972, 125169/1980, 73929/1980, 73930/1980, 42888/1982 and 1026/1982 disclose the technologies wherein the surfaces of magnetic powder are covered by oligomer or polymer each having a functional group that can be adsorbed to magnetic powder.
On the other hand, Japanese Patent O.P.I. Publication Nos. 23207/1975 and 22603/1975 disclose a copolymer as a processing agent for treating magnetic powder. This copolymer contains, ##STR1## as a unit, wherein a part of carboxyl group is changed to amide (--CONHR) and alkyl group (--R) therefrom gives a lipophilic property, thereby it is intended to enhance the compatibility of magnetic powder for the binder in kneading magnetic paints.
Recently, for the high-density recording, there have been proposed and put to practical use various technologies wherein magnetic metal powder which contains iron as its principal ingredient is employed. Magnetic metal powder is excellent in saturation magnetization and its coercive force is high and thereby its nature as a recording material is excellent. However, it has following two disadvantages because its surface activity is high.
(1) Anti-oxidization stability of magnetic metal powder in the air:
If magnetic metal powder is left alone in the air, its magnetic characteristic deteriorates gradually as its oxidization progresses and in the worst case, the spontaneous combustion may take place being caused by the external heat, shock and others.
(2) When magnetic metal powder are dispersed in the binder, the dispersion itself is difficult because of the high surface activity of the magnetic metal powder and in the worst case, even the gelation takes place.
For the above-mentioned reasons, there have been proposed some methods shown below from (1) to (5) as a method for stabilizing magnetic metal powder.
(1) A method for volatilizing the organic matter gradually after the magnetic metal powder are dipped in the organic solvent (Japanese Patent O.P.I. Publication Nos. 97738/1974 and 54998/1977)
(2) A method wherein magnetic metal powder and higher fatty acid salt are agitatedly mixed and thereby the film containing higher fatty acid group is formed on the surface of a particle (Japanese Patent O.P.I. Publication No. 97738/1974)
(3) A method for processing magnetic metal powder in the atmosphere wherein the oxygen pressure is adjusted (Japanese Patent O.P.I. Publication No. 7153/1971)
(4) A method wherein magnetic metal powder are dipped in an aqueous solution and then the surface of a particle thereof is compulsorily oxidized by an oxidizing agent (KMnO.sub.4 and others) (Japanese Patent O.P.I. Publication Nos. 30707/1981 and 69859/1977)
(5) A method wherein magnetic metal powder are dipped in a solvent (water, organic matter) and then they undergo a bubbling treatment performed by oxygen-containing gas (Japanese Patent O.P.I. Publication Nos. 85054/1977 and 16601/1981)
However, aforesaid methods all have following disadvantages.
In the method (1), it is possible to overcome the disadvantage (1) mentioned above but it is not good on the point of dispersibility.
The method (2) is effective against above disadvantages (1) and (2) but its reaction is not uniform because metallic salts which are insoluble in organic solvent are used.
The method (3) is feasible on a laboratory basis but it is not feasible on a manufacturing basis and aforesaid disadvantage (2) will not be improved.
The method (4) is effective for the aforesaid disadvantage (1) but it is difficult to control the reaction and the magnetic characteristic deteriorates greatly because of the processing in the water system.
In the method (5), the bubbling with gas makes it difficult to obtain the uniform oxidization and it is difficult to control the product, thus the method is not sufficient on the point of aforesaid disadvantage (2).